The invention relates to an illumination system for a surgical microscope, and more particularly to an illumination system which includes switchable illumination sources for use of the surgical microscope in eye surgery.
In the use of surgical microscopes in eye surgery, in particular in cataract extraction, the angle between the illumination axis and the observation axis of the microscope is to be kept as small as possible. An advantage of this type of illumination is that the light rays falling perpendicularly onto the eye are diffusely reflected by the retina and, as a result, the crystalline lens capsule of the eye appears as a reddish transmitted light. This effect is commonly referred to as a "red reflex". The quality of this red reflex is of particular significance in cataract extraction. In this operation, after initial removal of the lens of the eye all of the remaining tissue must be removed from the eye. This can only be accomplished if the remaining tissue can be accurately viewed, that is, illuminated with adequate optical contrast.
In the course of the operation, however, not only is the red-reflex representation required, but in addition, a conventional illumination, preferably a Kohler illumination, must also be used in order to illuminate the operating area Kohler illumination is a method of illuminating objects in which an image of the source is projected by a collector lens into the plane of the aperture diaphragm in the front focal plane of the condensers The condenser, in turn, projects an image of an illuminated field diaphragm at the opening of the collector into the object plane. In epi-microscopy (where the objective also serves as its own condenser) an aperture diaphragm is imaged by a relay lens into the back focal plane of the objective and the illuminated field diaphragm is arranged to be in a plane conjugate with that of the collector.
German Utility Model Patent DE 92 17 517 U1 describes one conventional surgical microscope with an illumination system by which a red-reflex representation of the crystalline lens capsule is made possible. In the described system, to increase the contrast for enhancing observation of a possible ametropia affecting the patient, an additional lens is swiveled into the beam between the main objective and close to the patient's eye. The illumination system described in this document has the disadvantage that the swiveling in of an additional lens between the operating site and the main objective reduces the operating space and consequently hinders the freedom of movement of the operating surgeon. Also, the complicated mechanical design means that there is no quick and uncomplicated switching over between the illumination of the operating area and the optimum creation of red reflex
German published patent application DE 40 28 605 A1 discloses a red-reflex illumination system for a surgical microscope. The red reflex is achieved by reflecting only certain, near-axis illumination rays from the illumination beam in the direction of the main objective via specially designed deflecting elements. In such a system, due to the blocking out of illumination rays, a high light loss must be tolerated.
U.S. Pat. No. 4,657,357 discloses another conventional illumination system for a surgical microscope in which the illumination rays are passed via prisms near to the axis through the main objective. For this purpose, however, it is necessary to provide the main objective with recesses which are complex to produce thereby increasing the expense of the microscope.
In general, in the known surgical microscopes for eye surgery, the pupil appears red in the observation beam if the cone of rays of the illumination and the cone of rays of the observation intersect. However, the observation of the red reflex is made more difficult by the brightness outside the pupil and by disturbing reflections of the image of the lamp filament on the patient's retina.